We plan to study the possible role of circulating hormones in causing sodium retention in rats with congestive heart failure. Cross circulation experiments will be carried out between 2 rats, one with heart failure due to a large A-V fistula and the other one normal. Salt and water reabsorption will be measured in the normal rat by micropuncture methods before, during and after cross circulation with the heart failure rat. Blood pressure, plasma protein concentrations and blood volume will be kept constant in the normal rat to exclude possible changes in Starling forces. If no evidence of a hormone is found, direct experiments on the heart failure rats will be conducted to study intrarenal Starling forces as a mechanism for sodium retention. Peritubular hydrostatic and colloid oncotic pressures will be measured, as well as distribution of filtration and blood flow among superficial and deep nephrons of the kidney. Experiments will also be carried out in isolated perfused kidneys of rats during systematic changes in either perfusion pressure or protein concentration of the perfusate. These experiments are designed to eliminate autonomic nervous system and extra-renal hormonal effects on the kidney. The extent of regulation of salt and water reabsorption by altered Starling forces will be evaluated under these conditions.